The present invention relates to the art of electrochemical cells, and more particularly, to a new and improved separator envelope for accommodating the swelling of a cathode electrode in high energy density electrochemical cell.
During normal discharge, the cathode of a lithium electrochemical cell, including but not limited to Li/SVO and Li/CFx cells, exhibits swelling. The separator, which is used to prevent physical contact between the anode and the cathode, adjusts to the swelling while maintaining structural integrity. Accordingly, conventional separator envelopes are fabricated with excess material distributed evenly around and about the volume of the covered electrode. This type of design is adequate when there is substantial clearance between the covered electrode and the header or lid of the cell. However, when the covered electrode and the case lid are in a closely spaced relationship, the separator may melt or become compromised during welding operations as the lid is being hermetically sealed to the rest of the casing. This happens because the casing and the lid readily conduct heat to the separator proximate the welding zone.
Accordingly, what is needed is a separator envelope that is suitable for cells having minimal or a closely spaced clearance between the electrode assembly and the inside surface of the case, and especially in areas that are near welding zones and which maintains its structural integrity throughout the discharge life of the cell.
Generally described, the present invention meets the above-mentioned need by providing a separator envelope that is characterized by a non-uniform offset from the covered electrode. This non-uniformity refers to the clearance around the entire body of the electrode. In other words, the clearance is not equidistant between the separator envelope and the covered electrode. Excess separator material is disposed in areas where there is adequate clearance from casing weld zones while the separator is in a relatively closely spaced relationship with the covered electrode in areas where there is less clearance from weld zones, for example proximate the casing lid.
In a preferred embodiment of the present invention, a casing having substantially parallel side walls connected by a curved end wall receives a cathode electrode having substantially parallel opposed side walls connected by end walls. In another preferred embodiment, the casing has a cylindrical shape housing a jellyroll electrode assembly. In either case, the cathode electrode is covered by a separator envelope that conforms in a relatively closely spaced relationship to the contour of the electrode body, including adjacent to the casing weld zones, such as adjacent to the lid. Distant from the weld zones, the separator envelope has excess material and is in a relatively loosely spaced relationship with the covered electrode. This loosely spaced relationship accommodates electrode swelling during cell discharge. Accordingly, the separator envelope of the present invention is preferably spaced from the covered electrode in a non-uniform manner. While the separator configuration of the present invention has been described with respect to the cathode of a primary cell, it is equally applicable for both the anode electrode and the cathode electrode of a secondary or rechargeable cell.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and to the appended drawings.